[TF-M] TF-M v2.1.5 release announcement

Hello,

I am pleased to announce the release of TF-M v2.1.5.

Major highlights:

* SPM: Fixes for FPU context cleanup, add r12 into caller context clearing * Upgrade to MbedTLS v3.6.6, which contains security advisories.

Please see the release noteshttps://review.trustedfirmware.org/c/TF-M/trusted-firmware-m/+/50758/2/docs/releases/2.1.5.rst for full details.

Many thanks to Nicola Mazzucato for leading this step, and to everyone who supported this milestone.

Best regards, Anton